Saturday, December 17, 2016

In my first post on the subject of Crossover Basics we talked about the electrical response. How various types of high and low-pass filters changed the voltage that the speaker drivers experience. We glossed over the issue of impedance entirely. Understanding this will help you better grasp what's going on.

Introduction to Speaker Impedance

Anyone
who has examined speaker specifications has seen an impedance rating,
such as 4 or 8 Ohms. If you see speaker measurements from Stereophile or
SoundStage you would have come across charts showing complicated
details about impedance and phase. Here is the impedance chart from the
ported LM-1. This first chart shows the combined effects of the drivers,
crossover components as well as the cabinet and port.

Ignore the thin blue line, and focus on the thick one instead. Notice
the chart axis. Across the X (bottom) we have the frequency scale,
across Y (left side) we have Ohms. So the first thing you learn is that
the nominal 4 or 8 Ohms you are used to seeing are kind of a summary,
and sometimes an outright lie. No speaker is exactly 4 or 8. These are
guides to help match speakers and speaker counts to amplifiers.
Personally I would rate the LM-1 as an 8 Ohm speaker, though it dips
below this above the bass. The triple hump you see is indicative of a
2-way, ported loudspeaker. The two humps at left are caused by the port,
while the hump at right, just below 2kHz is where the high and
low-pass filters meet.

You might have picked up on the fact that we are using Ohms to measure
impedance (sometimes called Z). What makes impedance different from Ohms
is that it is frequency dependent, and it has an angle. This angle is
the difference between the voltage and current. We'll ignore this for
now and focus on the magnitude in Ohms. Impedance is what we use to
understand the effects of capacitors, coils and speaker drivers which
are electrically complicated little beasts.

One
very important point to note is that even though the tweeter and
woofer sections are arranged in parallel, the impedance of the drivers
does not actually appear in parallel. That is, a pair of 8 Ohm drivers
wired in parallel would normally result in an impedance of 4 ohms, but
in this "perfect" system the actual impedance stays very close to 8 Ohms
at all frequencies.

Let's talk a little more about the LM-1, above. From left to right, the first two humps are classic
indicators of a ported loudspeaker. From the bottom up to around 1,000
Hz the impedance is entirely that of the woofer. The peak around 1,800
Hz occurs where the low and high pass filters overlap, and after about
5kHz, the impedance is entirely a result of the tweeter.

In the sections that follow we'll go over each section in detail.

Effects of High and Low Pass Filters

We'll use the original example of a first-order crossover for all of our discussions.

To refresh your memory, this chart shows us how first order filters behave. The red trace is the electrical signal a tweeter would see, while the yellow is the woofer (1 kHz is actually a very low frequency for most tweeters, let's pretend it's OK). Black is the summed response of the tweeter and woofer. While it is smooth and straight we don't care about it right now.

This is strictly the voltage and electrical view of what would be measured at the inputs of the "ideal" 8 Ohm tweeter. We are, of course, missing the acoustical results, but we cover in the post Crossover Basics - Driver Response.

The question we answer in this post is: "How do we achieve this frequency dependent change in response?" Let's take a look at the original crossover first:

There is a capacitor in series with the tweeter, 20uF. We talked about it "blocking" low frequencies. This is achieved by an increased impedance. That is, as the frequencies drop, the Z (impedance) of C1 goes upwards. Let's take a look at the impedance of the tweeter circuit, with the driver and the driver + capacitor.

Series Voltage and Impedance

Keep this rule in mind: The voltages across each component in a series circuit is proportional to the resistance of each component. We're dealing with impedance, and complicated but for the sake of simplicity we ignore it here. This simple rule-of-thumb is enough to explain the behaviors without delving into reactive impedance calculation.

High-Pass Impedance

In the chart below the red line represents our tweeter impedance. Again, this is theoretically perfect 8 Ohms. No drivers are like that in rea life but we need it simple so we can better see what the high-pass filter is doing. The blue line however represents the entire tweeter section. That is, C1, S1.

Unfortunately at 100 Hz the impedance is cut off but it's close to 80 Ohms. If C1 is 80Ohms we can then estimate the relative voltages across C1 and R1. It's only an estimate because we're not taking phase into account, but it's going to be very close. So we estimate the voltage at S1 to be 8 / 80 = 0.1 of the total. So at 100 Hz the tweeter gets approximately 10% of the voltage. At 10 kHz Z ~= S1, meaning our tweeter gets nearly all the voltage, and C1 is acting like a short circuit.

With those two points (100 Hz and 10 kHz) in mind, now the following graph should make sense. It plots the voltage across C1 and S1 as the frequency varies. Assume a 4V input voltage.

As we calculated, at 100 Hz the voltage across S1 is about 4V x 0.1 = 0.4V

This shifting of impedance and voltage between the filters and the drivers is similar regardless of the order of the filter. It's also mirror-imaged for the woofer.

From Voltage to Decibels

Let's calculate the dB drop at the crossover point of 1 kHz. The peak tweeter input is 4V. At the crossover point it is approximately 2.8V. So, we calculate using the rule:

dB = 20 log ( Vout / Vin ) = 20 log ( 2.8/4 ) = -3 dB

In other words, at the crossover point the input to the tweeter is 3 dB less than the amplifier output. Simple, right? let's do this again for 100 Hz:

20 log ( 0.4/4) = 20 log ( 0.1 ) = -20 dB

So at 100 Hz, the voltage to the tweeter is -20 dB below the amplifier's output. A very good thing since tweeters are easily damaged by low frequency signals.

Low-Pass Impedance

The same and complementary effect is happening at the woofer side of this example:

In the woofer side, by around 100 Hz L1 has no meaningful contribution to the impedance magnitude (i.e. Ohms) so nearly 100% of the input voltage is at the woofer. By 10 kHz however the impedance of the coil is approximately 80 Ohms. Estimating 8/80 = 10%.

Additional Exercises

Using XSim, and "ideal" drivers, create 2nd order filters and compare the impedance of the high and low pass sections. Compare the electrical response and impedance curves. Combine them and examine the impedance bump where they meet.

Using a second order filter, examine why the 2nd component works without shorting out the entire speaker. For instance, if a high pass filter, examine why the coil (L) works. What would the impedance be if the coil alone was there, but not the capacitor?

Thursday, December 8, 2016

Introduction to the Zobel

A Zobel network is used to flatten a driver's impedance (usually a woofer or mid-range), therefore making the filter (usually low-pass) more effective. A Zobel consists of a capacitor and resistor which are wired in parallel to a driver.

C1,R1 on the left are an example of the Zobel network. Their values are chosen to minimize the impedance rise of a driver above the resonant frequency.

Speaker Impedance

Introduction to Series Circuits

Take a look at a very simple series circuit. It consists of two resistors in series (one after another) with the amplifier. The circuit is closed by the ground points (the downward facing triangles).

As before, this may be something you like to play with, so I encourage you to grab a copy of XSim and try this out along with the Peak Voltage chart.

What's important to understanding here is that the voltage across the resistors will be proportional to the resistance offered. So

Vr1 = Vin * R1/(R1 + R2)
Vr2 = Vin * R2/(R1 + R2)

And of course:

Vr1 + Vr2 = Vin

That is, the voltage across R1 and R2 must add up to the input voltage.

Think of Ohms as elephants that eat volts. The more elephants, the larger portion of the incoming voltage they eat. Yes, this is a very silly analogy. Still, we can do some quick math. The total resistance is 100 Elephants (hah!) or 100 Ohms. R1 has only 10 elephants, so it gets 10% of the incoming voltage, whatever that voltage may be. R2 has 90% of the Elephants, so it takes 90% of the incoming voltage. There's a lot more to circuit analysis, but this is the bare minimum to understanding Zobel networks. Hopefully you'll be intrigued and learn more on your own.

Woofer Impedance

So with a little background under your belt you are now ready to look at a typical woofer. We'll use the LM-1 woofer, the Peerless 830991. You should know that impedance graphs will change once a driver is in the cabinet, especially if the cabinet is ported, so the data I present here will be different than a specification sheet which measures the driver in free-air.

You have heard the term "coil" used interchangeably with
"inductor." Which is correct, but you may not have thought about the
term "voice coil" in the same context. The voice coil is the part of the
speaker that will electrically connect to your amplifier and produces
the magnetic force which moves it against the magnetic field of the
permanent magnet. We won't get too much into this, but suffice it to say
that above the resonant peak the voice coil behaves like that of any
other inductor, specifically it has both a resistive element (DC
Resistance, or Re) and an inductive element (Le). These combine to give
us the woofer's electrical impedance (Z) at any given frequency.

In the chart below we will compare the impedance of the woofer in a sealed cabinet (red) with a woofer that has a Zobel network applied (blue).

Let's ignore what happens below 200 Hz. That's a topic beyond this posting, and it's also far below our likely filter points. From about 200 Hz to 400 Hz the woofer is purely resistant, and we have the minimum around 6.6 Ohms, but what happens to the right? That's correct, suddenly more voltage-eating elephants arrive! The inductive qualities of the voice coil become more and more important and overwhelm the well behaved resistance. Compare the peak difference of the two impedance charts, all the way at the right. The blue line represents a woofer compensated with a Zobel network. The impedance never goes above 7 Ohms, while the normal un-compensated woofer goes to over 30! That's more than 4:1 difference. This increase impedance is going to compete with the low pass filter and make it behave in ways we probably don't want.

For clarity we'll leave behind the LM-1 schematic and create a new one, with two identical woofers and 2nd-order low-pass filters set to 4 kHz. Of course, this is not how a real speaker would be designed, we are just using this to see exactly how a Zobel circuit works. The Zobel consists of C1 and R1.

The first thing we should do is examine the transfer function of the two filter sections:

As you can see, S2 is behaving like we expect a low pass filter to work. S1 however is having a very difficult time getting to the right slope. At 5kHz the output is almost 10 dB higher than we want it to be. That's a big deal. Eventually the impedance (elephants) on the low pas filter take over, but they don't reach our desired behavior until past 20 kHz, definitely not good enough for us. Let's take a look at the final outcome, below:

How Does a Zobel Work?

Above we discussed how serial components work in a circuit. You may feel a little tricked because while we learned enough to understand why coil inductance needs to be compensated for, we never talked about how a parallel circuit works, which is what a Zobel is. A parallel circuit has one unique property:

The apparent impedance of a parallel section is never more than the smallest impedance.

If we imagine C1 as a short, then no matter what S2 rises to, the impedance will never go above R1, or 8.2 Ohms. It can be less than that, but never more. In a parallel circuit you calculate the apparent impedance like so:

Rtotal = 1 / ( (1/R1) + (1/R2) + (and so on and so forth) )

Things are more complicated because we are actually calculating impedance, but you get the picture.

Let's do some quick, Dr. Leach style of analysis on the components in a Zobel. At very low frequencies, C1 behaves like an open circuit, essentially removing R1 and C1 from meaningful contributions to the system impedance. Remember we mentioned that impedance cannot rise more than the smallest value? So at low frequencies C1 is so large that S2 becomes the limit on impedance. You can see the impedance below 500 Hz or so is barely affected. At high frequencies, C1 rapidly decreases until it evectively becomes a short, putting R1 in parallel with the driver, S2 and limiting the absolute maximum impedance to 8.2 Ohms. In this case we don't reach 8.2 until well-past 20kHz but it would eventually reach that point once the woofer's impedance was high enough. Also past our point of concern. If we can limit the impedance from 6.6 Ohms to 7 Ohms then we have a much more stable impedance curve than before, and that's good enough.

Do I Need a Zobel?

That's a tricky question! So let's examine this woofer and it's output. If you wanted to cross it over at 4kHz I would think the Zobel was mandatory, however if you were going to set your crossover frequency at 2kHz or lower I would say not really. The LM-1 uses it, but the effect of the Zobel is small, and benefits the phase response so I leave it in. It is possible a very similar sounding LM-1 could be built without a Zobel and with different choices in the filters left behind. The best chart to look at to see if a Zobel matters in your circuit is the transfer function chart.

It is very rare, but not unheard of, that a tweeter needs a Zobel because their voice coils are relatively tiny and therefore don't have a lot of inductance. The most common exception to this rule is with ribbon tweeters. The ribbon itself is not inductive but the entire assembly often include matching transformers. Transformers are coils .... and coils are inductive... see where this goes? :)

The real point to the Zobel is to make things better in the area you need the filter to behave at it's best. That's usually up to about -20 to -30 dB. Beyond that if your slope isn't perfect we no longer really care. There's no audible difference between -60 and -67 dB for example.

An important consideration in choosing a Zobel or not is that they are not free. The more parts in a system, the more expensive, the more chances of failures or parts being out of specification. If this is a personal project, no problem, it's all experience. However if you are building for mass production eliminating unnecessary components is the final stage before committing a design to the factory.

Placement

In most cases, you want to place a Zobel closest to the driver. Put anything else such as padding resistors, filters, etc. before it. While the order of serial components does not matter, the order of parallel components does. Leaving the Zobel last prevents unexpected consequences.

There is a rare exception, when you must equalize a driver (usually a tweeter) by adding inductance. In which case you want the equalizing circuit closest to the tweeter, then the Zobel, so the Zobel can also control the EQ's impedance. I'll write more about this later in a section on handling difficult tweeters.

The Secret Uses of the Zobel

Many will rely on on-line calculators to determine the right values for a Zobel network, and that's fine, but be aware that the absolute values can be tweaked. The main benefit of this tweaking is to gently nudge the phase charts one way or the other, helping you get near-perfect matching between two drivers you otherwise might not have. This is where having a tool like Xsim to simulate your tweaks comes in super handy.

Exercises

The Peerless 830991 has an Re of around 6.6 and Le of around 0.330mH. Try simulating this in Xsim using a resistor and coil in series. Compare your impedance curve with the red impedance curve, above. What's the biggest difference you see?

Try using an online-calculator to create a Zobel for this driver. Tweak the capacitor and resistor values. Can you do better than the on-line calculator?

Using the complete LM-1 schematics, compare the woofer response with and without the Zobel. Is it a big difference? Can you fix the LM-1 so it no longer needs a Zobel? What difficulties did you encounter?Pay attention to the phase matching as well as the frequency response.

In the article published by audioXpress D'Appolito shares an interaction with Stereophile head honch John Atkinson (JA). JA did something I though was pretty interesting mathematically, but I call bullshit on his message. He claims he analyzed a number of speakers and compared them to those which would make the recommended components according to frequency response and that most were perfectly neutral. D'Appolito states:

[John Atkinson] defined the standard deviation (SD) from flat response over the
frequency range of 170Hz to 17kHz as a criterion for judging flatness of
frequency response.

Further:

Of the 15 speakers with an SD of 1dB or less, 14 were added to the list by Stereophile reviewers.

So, bunk. I personally don't care what John Atkinson likes. If he likes the B&W diamonds above all others that's fine with me. But to call them neutral, or try to sell them as the reference against which other speakers are too dull or bright is shilling.

Sunday, December 4, 2016

The Decibel or dB

Decibels (dBs) are a curious way to measure electrical and acoustic energy. Curious, and terribly convenient! For us, we use relative electrical dBs to discuss how filters work, and absolute acoustic dBSPL to measure speaker output.

When discussing the effects of a filter on a signal, we'll use relative dBs. That is, there's no set standard, but we talk about something being +4dB or -18 dB. This is useful because we can map this to speaker outputs no matter the volume settings. It is how we will discuss how a filter works, without worrying about the absolute output levels.

On the other hand, when we discuss the acoustical outputs we'll use dBSPLs which are in absolute terms, but using a set input level. Don't worry too much if this is confusing, we'll make it more clear as we go along.

The LM-1 Crossover Revisited

We are going to use the LM-1 crossover and focus on the tweeter response in detail. Let's refresh your memory about the crossover, here it is on the left.

We'll focus on the tweeter filter section. This includes C1, L1, R1, and R2. The woofer section will seem neglected by comparison, but we cover it in more detail in other blog posts, including the Zobel.

Let's go over the transfer function. That is, how the voltage at the tweeter is different from the amplifier output because of the crossover. 0 dB means there was no change, the input and output are the same. The woofer response (in red, below) is almost exactly 0 dB until around 700 Hz when the low-pass filter kicks in. The tweeter on the other hand is more complicated. Let's discuss.

Anytime you see a chart this clean, you can be sure you are NOT looking at acoustical measurements. The blue line is tne tweeter filter's response. Except for the level shifting, this seems like something straight from my previous post on Crossover Basics. First, notice the level of the tweeter. It has been "padded" or "lowered" 6 dB below input. This is accomplished by the 4.2 Ohm R1. R1 is effective at all frequencies. Everything gets shifted down about 6 dB because of it. It's not exactly always constant, but let's pretend it is for right now, which is very close to true.

In addition to the padding there is a high pass filter reducing the midrange and bass at about 8 dB/octave below 2kHz. We discuss pads by an absolute number, like "6 dB" because it's effect is constant at all frequencies but we talk about high and low-pass filters with rates. In this case, 8 dB/octave means every time you cut the frequency in half, you will loose 8 dB. This is the actual "high-pass" section at work. This is C1,L1,R2. Notice that after about 4 kHz the high pass filter effectively stops working. It's as if it wasn't there anymore. Above this level the only parts still involved in the high frequency response are the tweeter and R1.

Putting it All Together

The point of this post is that these changes are not in isolation, but rather in combination with the driver so let's take a look at how the padding resistor and thigh high pass filter combine withe the acoustical response of the driver to produce the final outcome.

Notice the scale is now different. We are now looking at dBSPL, or sound pressure dBs. It is most common to take the frequency response measurements of a driver at 2.83 Volts input with the microphone at 1 meter distance. As you can see, below, this particular tweeter outputs about 90 dB at 2.83 volts above 4kHz or so. 2.83V is a common reference standard because at 8 Ohms this is about 1 Watt.

The top black line represents the tweeter with no filter at all. The green line represents the tweeter with just R1 added. It's not exactly 6 dB down everywhere due to the tweeter's impedance curve, but it's close enough for us! You'll learn more about this in the next post which covers the Zobel. The red line represents the addition of the high pass filter section, C1, L1 and R2. You can see it pivots around 3 kHz.

By carefully selecting the filter knee (-6dB point) and it's Q, or steepness we can get a little bit of EQ thrown in for free. Take a look at the original response (black) at around 2 kHz. You see the broad bump centered there? The bump is pretty much gone thanks to the high pass filter. We have not only added the high pass filtering, but we also tamed a little over-activeness int he tweeter without increasing the part count.

Padding

In the chart below you can see the final LM-1 design in red, vs. the a redesign without R1:

It may not be obvious from this, especially since this author likes to use far-field as his reference, but the LM-1 without padding would shriek.

In designing a crossover, I find it easiest to start low and work my way up. The low pass filter will reduce the sensitivity of the woofer at the crossover point. After this, we must adjust the tweeter to match and then add the high pass filter.

The total amount of padding (dB loss) depends on a number of things, including:

Innate woofer efficiency

Woofer low pass filter and baffle step compensation

Innate tweeter efficiency

Tweeter high pass filter

Unfortunately there is no simple, accurate way to go from a manufacturer's sensitivity specs to appropriate filter design.

The crossover designer must balance all four of these issues at the same time which is why in-cabinet measurement and simulation are so important. I encourage you to grab the LM-1 simulation files and attempt this for yourself.

Also note, that doing the reverse, padding the woofer, is generally discouraged because the power dissipation needed to lower a woofer a few dB is pretty large and requires big resistors and will waste a larger amount of amplifier energy. If your tweeter is too insensitive you probably need to change tweeter or woofer. It is pretty rare to find any design that does not require any tweeter padding.

Summary

With this posting, you now have learned:

How crossover filter's add to driver output to create the combined effect of both.

How you can use leverage a high pass filter to also work as an EQ for you.

Why tweeters usually have resistors to pad them down.

In my next post, Crossover Basics - The Zobel, we'll go over the LM-1 woofer response but spend particular attention on the often misused or misunderstood circuit, the Zobel.

Monday, November 14, 2016

I've come across a couple of incredible deals over at Parts Express I hope to have the time and money to take advantage of. As you may know, I've been listening to the LM-1 as desktop speakers. They do a fantastic job with just 20 watts, but.... they really have no bass. It shows up more on games than movies.

The LM-1's have amazing bass in a bookshelf with very good rear wall reinforcement though!

I've been using the LM-1 with a full-range 20 Watt digital amplifier and they really sound great. I expect the 2.1 amplifier to sound even better. The plate amplifier adds a high-pass to the amp. So the speakers will still have 20 Watts, but dedicated to 80 Hz on up. A separate 50 Watt amp drives the subwoofer itself. See where this is going? Of course, everything depends, but you could end up with the same volume and power of a s150 Watt/channel system. Bi-amplified systems are more power efficient than single amp systems, generally speaking and with music so this idea makes mathematical sense.

The combination is the perfect size for a desk or dorm-room. I just hope the cash magically appears so I can build it before the sale ends.

Monday, October 31, 2016

Unfortunately it seems my designs are so popular that in the US the LM-1 and its sibling the LM-1C have caused Madisound and Parts-Express to run out of stock. Fear not, Parts-Express expects more in February of 2017.

If you absolutely cannot wait, the Peerless 830860 will substitute. Same motor with a polypropelyne cone. Probably won't be as transparent, but I've not heard or measured it. Specs say it's going to be very close.

Thursday, October 27, 2016

I purchased a Lepai 2020A 20 watt micro amplifier to power the LM-1 speakers I recently put on my desktop.I used the micro plug for the PC sound and the RCA inputs are being fed from my Logitech Squeezebox Touch.

The Lepai 2020A is part of a family of amps that visually look identically but are based on different chipsets or have higher power ratings. The "A" variant is based on a Yamaha chipset. Yamaha has been making professional amplifiers for decades, including very high efficiency models for studio and touring use. The 2020A uses a 12V/3A supply.

Based on a variety of reading material, I doubt the 2020A does 20 watts/channel. It's probably closer to 7 when you limit power rating to 1% THD. Still, sounds very good.

The sound is smooth, very quiet, and not fatiguing at all. It has plenty of power for watching Hulu while in bed or playing games, as well as listening to Jazz FM 91 via the Internet.

The tone controls are subtle! They may not do it for you if you are looking for big changes, but if you like to gently adjust bass and treble they'll be great for you.

Some users report even better performance with beefier power supplies which I did not try. In my case small and hidden is key! Plus I never really felt the amp was too small. One thing though, the LM-1 speakers are VERY easy to drive. Your success with speakers that have lower impedance may not be as good.

The Lepai 2020A does not have any input switching. Both the RCA and mini jack inputs are live at all times. Sadly they are mixed through a resistor, not buffered. and then mixed together. This means that if two devices are operating, you'll get half the volume of either. Turn one off and the volume doubles.

If you don't intend to head bang, but are looking instead for a solid mini-amp to put under your desk, or in a boat or RV, I highly recommend the Lepai 2020A.

The amp runs ice cold at all times. The one negative thing is this desire for Asian manufacturers to put blue LED's on every damn thing. In this case it's around the volume control. It's big, bright and blue, so you can forget about sleeping with it on nearby.

Friday, September 30, 2016

This is kind of a follow up on my original article Stereophile - The Data Doesn't Lie and, to be honest, this is a tempest in a teacup as well. It's just one sentence that lit a fire under me.

First, I've never heard the Arabesque Minissimo, nor can I really morally justify the purchase of small home speakers that cost $20,000, whether from Crystal or Magico.

Even then I am nothing if not a hero for truth and justice so it was inevitable that the latest online review from Stereophile would completely piss me off. For the most part, it's a too crappy and short for a pair of speakers with such pedigree.

Stereophile Listening Tests

The listening section is only one page, and very little devoted to
actual music listening. Also, John Atkinson seems to write that he
relies on test tones and his ears to tell "tonality" instead of oh, I
don't know, music? I mean, your ears are for music, not test tones. If
you have the gear, you don't need to be using test tones and aural
guessing.

The biggest problem is that by JA's own admission, the speakers could not be placed optimally in his room. Instead of realizing there was significant "operator error" JA goes on to blame the speaker's and claims "treble tailoring" will only play nice with some music.

So what happened? Speaker/reviewer mismatch.

To have the maximum possible sensitivity and simplest crossover it seems that the Minissimo foregoes "baffle-step compensation" or BSC, and requires close wall placement. I have highlighted this region under the line in the chart at the left. This rising bass effect happens as the woofer's output goes from omni-directional to half space, and is related to the woofer and the baffle width. Notice it does not show up in his near field graphs...that's a whole other story of mess too long for this post.

You might look at that and say "what a crappy speaker!" but you'd be wrong. There's nothing wrong with designing a speaker for a specific environment, and we need more speakers that don't try to dominate your living room. This kind of design approach can yield exceptional results. Unlike almost all "high end" speakers, the Minissimo's were never designed to be a "one size fits all" model. They are ideal for a salon where discretely
beautiful speakers could play wonderfully while leaving space for
everything else. Listening to the Minissimo's in the middle of the room will sound like a mosquito orchestra, which seems to be just what happened.

JA rigidly reviewed them as middle-of-the-room stand mounts and blames
the speaker when he should be blaming his closed mind and lack of
understanding. With these measurements in hand, JA should have stopped or found a better place to listen to the Minissimos, or held of on publishing the review. Instead he attributed the weak response to the "tailored treble." His excuse that his furniture did not allow him to place the speakers correctly is the height of professional whinery. Either move them to the right place, or let some one who can and wants to them properly so. Don't compromise by writing a half-ass review with the speakers in a poor location.

The Bottom Line

The Crystal Cable Minissimo designers took truly refined components with a minimalist design aesthetic and made a high quality, beautiful looking pair that will stay out of your living space. Comparing the Minissimo to the slightly less expensive, but equally insensitive, Magico S1 Mk. II, the S1 requires much more floor space to sound it's best. To me, a small speaker that actually needs so much space is kind of a weird situation. On the other hand, the Minissimo is small (for a speaker with a 6" woofer) and perfectly happy to get cozy to the art on the wall.

With this in mind, if you are a music lover who wants top tier, beautiful speakers made from rare materials that don't take over your room I strongly encourage you to listen to them for yourself and pay no attention to the critic behind the curtain.

Tuesday, June 7, 2016

As a side effect of the movers taking most of my furniture, I had to move the LM-1s off their stands 2 feet from the rear wall to half that distance, sitting on books on the TV stand.

In this location I find them too boomy. What's the solution? Socks. I put a big clean fluffy white sock to plug the port in the back of each. Problem solved! They don't sound as good as they did further away, and further apart, but they have regained their tonal composure.

Sunday, June 5, 2016

A number of readers have asked me about the horizontal layout of the drivers. I was just being creative with the pictures, but I personally listen to the LM-1's with the traditional tweeter-over-woofer, "portrait mode" arrangement. Of course, you can lie them down to fit tight spaces, or to keep a lower profile over a mixing console, but I am not intentionally designing "landscape mode" speakers.

The LM-1 has a wide and natural dispersion but is best with the tweeters on the outside and either parallel or toed in. Vertically however the LM-1 sounds best with your ears at tweeter level or lower. Any higher and it sounds dull and lifeless.

The LM-1C is similar. Used vertically keep the tweeters to the outside of center, toed in slightly. Around 15 degrees. This will give you a big sweet spot in the center, and limit reflections to the side. Used horizontally keep the tweeter pointed at or slightly above the tweeter towards the listener location, You may wish to experiment with the tweeter on the "outside" of the TV. In other words, keep the woofers closer to the TV than the tweeter but tilt the speaker vertically. Play around with it a little and the best tuning will become pretty obvious.

The LM-1 speaker kit has only been promoted for a little while and already it's evident there's a lot of misinformation and attempts to discredit the design. I hope it doesn't become the Kardashian (any of them) of speaker kits. In any event, one completely inaccurate criticism is that the frequency response is below-average. Nonsense. Could you use different parts and asymptotically approach perfection? Probably. You could get different, but getting noticeably better will be very difficult.

Let's go over some background.

Who is Bruel and Kjaer?

When scientists and engineers think of high-end acoustical instruments Bruel and Kjaer is among the most respected names there is. One famous bit of knowledge that has come out of that organization is the ideal B&K speaker curve, which I happen to like a lot (that's a personal preference). It is meant to be as the ideal response at the listening position. I copy it here, below:

Note that it is not flat! A speaker that measured flat at the listening location would be an ear drill. The B&K curve is about +3dB at 70 Hz or so and -3dB at 20 kHz. Quasi-anechoic measurements measure the speaker driver in near-field, with the expectation that at a distance the measurement would become like you see here. The full discussion of why this is beyond the scope of this posting, but do your research and you'll find much more written about it. You may also find that the Dirac Live target curves, also follow the B&K model of a gently descending response. The point is, I don't make this stuff up. Flat at the listening location is not actually ideal. What is ideal is open to some interpretation. I'm choosing the B&K curves as a matter of taste.

So, how does the LM-1 do by comparison to this fabled B&K curve in real life? Let's comapre (LM-1 level is offset and measured in a bookshelf):

Damn well! This is the plot with R1 = 4.2 Ohms (2.7 + 1.5 actually). I'm packing and that's all the resistors I have on hand.

So, what does this say? This is an objectively neutral speaker kit. If you are looking for a true bookshelf speaker system with reference grade frequency response the LM-1 will do it for you, within the limits of what a small monitor can do. Also, as discussed, the treble balance is up to the builder. If you don't like it, change it. :) I have a feeling that most listeners will like a value closer to 3.9 Ohms or so.

Saturday, June 4, 2016

My usual go-to medium grade capacitor is Mundorf MKP, but since I had such good results with the Clarity MR caps in my reference speakers I decided to try the Clarity ESA's. 24 hours into the experience and it's still too hard.

I've increased R1 from 2.7 to 4.2 Ohms and it's much better. The LM-1 curves match the center channel curves more closely. However I'm still not quite satisfied with the treble. Of course, this is a $35 ring radiator tweeter, vs. the $500 Mundorf AMT I've gotten used to. The two may never closely align, but I have hopes. I've heard lesser tweeters smooth out with better caps, and I've heard caps mellow out after a few days, so this may take a while before I'm ready to say I'm done.

June 5, 2016
At about 48 hours the caps got noticeably less harsh. It's also interesting to note that changing R1 from 2.7 to 4.2 made only about 1.6 dB of difference at 13 kHz, but it's all the difference in the world to my ears.

June 6, 2016
In spite of the packing going on around here, I closed some drapes and listened more last night. I'm ambivalent over the total treble balance, but the tweeter and C1 seem to have settled in quite nicely. I sometimes feel it may still be a little bright, but the measurements don't lie, I'm actually just under the B&K curve, so I'm going to leave it alone. Honestly I think my reference speakers are a little dark, so this may be a good chance for my own ears to adjust. My PC is getting packed up today, so I'll be unable to do more measurements or design as I won't have access to my only Windows 7 PC. This will be a good chance for me to explore what else is available on Ubuntu/Linux! :)

Introduction

For a long time I've had trouble matching up Stereophile reviews with my experience of the same speakers. I think I've found the reason why. They aren't reviewing speakers at all. They are reviewing hearing aids pretending to be speakers. This is why they are so expensive. What I mean is that the speakers Stereophile praises would only sound good to some one with hearing loss between 7kHz and 15 kHz, which I lack. It's clear that completely different manufacturers have taken advantage of this "trick."

I'm calling this trick the "Stereophile Curve" and the more I go back in time to look at megabucks speakers rated highly, the more apparent the truth of this curve becomes. Of course, the alternate, benign explanation is that the reviewers have all bought the B&W studio heritage hype, and they have become accustomed to thinking of the B&W 800 series speakers as a neutral reference, which, objectively, they can't be.

The Stereophile Curve

A recent review in Stereophile for the B&W 802 D3 speakers prompted a reader (AV OCD I think) to post pictures showing just how similar the frequency response of the D2 was to the D3, despite the glowing reviews from Kalman Rubinson for the D3's superiority. The reader was right, the frequency response aligned nearly identically. Mind you, that's no proof of anything except that it's hard to match up the comments with the data. But as I looked more closely at AV-OCD's charts I realized that I had seen nearly identical measured response curve elsewhere, in the review for the Golden Ear Triton 5!

Several of the most highly touted speakers Stereophile that reviews not only have pretty ragged and, in my humble opinion, substandard frequency response, they are also ideal speakers for the hearing impaired. That's the nice way of saying, they are deliberately tweaked in the same way. Shenanigans say I!

Let's compare the B&W 802D, 802 D3 and the Golden Ear Triton 5:

See the dip around 6 kHz and the bump around 11 kHz? To me, having heard 2 of the three speakers in the chart above, this looks like the Marquis DeSade's setup. It also explains what the reviewers at Stereophile are raving about. There is no reason for this by the way except deliberate mis-tuning of the crossover from neutral or ideal. It also tells me this:

If you are using B&W 802 series speakers as references for record mastering or post-production, please stop it, stop it now! They aren't going to match most home speakers or any well-calibrated cinema.

Listen, if you like the sound of any of these speakers, and can afford them, you should go buy them, especially if you have complementary hearing loss but what you won't be able to do is to convince me that I'll like any pair of speakers Stereophile recommends, I just don't have the hearing loss in the same place that they seem to.

The Sony SS AR1 Review

The Stereophile review of this speaker was the most lukewarm review I've
seen them publish for a $20,000 speaker. What happened? The frequency
response is not what is in vogue. Take a look at the Sony vs. the
B&W 802 D3 measurements:

The B&W 803 D3 measurements are in red. I've circled all the
important parts of the "Stereophile Curve" in red. As you can see, the
Sony AR1 misses all the key parts. From the left, the 2.4kHz dip i s a known imaging/depth enhancment. It makes speakers appear to have better imaging than the recording actually has. Several Wilson's are tuned with this as are the Focal Sopra 2s. After that it seems to be a trait of the B&W's, regardless of their midrange. By the way, that Golden Ear was able to match B&W's curve so well is a testament to the versatility of AMT diaphrams. Being mostly resistive, creating an EQ curve to match other speakers is much easier.

Mind you, I'm not really happy with the Sony AR1 response anyway, it's
not neutral but more of a party tuned speaker. My point is that Sony
did not know about the current fads in the high-end, and if they had they would have had a
much warmer reception.

Exceptions to the Curve

There are exception to the Stereophile Curve, such as the Wilson Audio speakers, which
just have terrible and inconsistent high frequency response. The Wilson XLF did seem to follow this curve though. YG Acoustics is one of the
rare exceptions that proves the rule. Exceptionally good frequency
response. Never heard them.

Explanations? Not sure, but they seem to correlate well with the number of full page ads. Take a look through any show report in the last 5 years at Stereophile. I'll let you punch me in the arm for any show report that doesn't include a painfully boring picture of both a Wilson AND YG Acoustics demo room.

Alternatives to the High End Speaker Hype

The speakers that I like to listen to neutral and gimmick free, at least until my hearing gives out. If you feel the same way, please help stop the madness by buying from custom makers or by building your own speakers.

Please feel free to take a look at any of the good kits mentioned in this blog or my blog page which focuses on Custom Speaker Making, here.

Hearing Loss

By the way, if you do have hearing loss, building speakers, or adding equalization to make speakers sound better for you is quite easily and cheaply done! Dan D'Agostino, founder of Krell and now his own company has had hearing aids specially tuned for him for years. He seems to have coped quite nicely! There's no reason why you can't remain a music and audiophile in the 21st century even with moderate hearing loss. miniDSP is your friend!

This blog post is not out to berate some one for a health condition. It may however berate an entire industry for selling hearing aids masquerading as loudspeakers for tens of thousands of dollars or more. Hearing aids, whether the kind you wear in your ear, or when disguised as loudspeakers should be affordable for all, and reviewers should know better.

Friday, May 27, 2016

The part list for the LM-1 Bbookshelf Version is here. The only things missing is wiring and crimp connectors as well as adhesives and circuit board surfaces.

In the US Parts Express usually has all of these parts, especially the hardware but Madisound often has better driver prices. For the crossover parts I buy a lot of Mundorf MKP, Mills and Clarity ESA caps so I tend to buy most of those from Parts Connexion except for the Jantzen coils which always come from Parts Express.

For a detailed discussion about the crossover components and drivers please see the blog entry titled LM-1 Bookshelf Crossover.

Update: August 13, 2017: The Denovo flat-pack cabinets are about $50/pair vs. $190/pair for the Dayton cherry cabinets. The Denovo is about 1/2" shorter so plan on squeezing the woofer and tweeter together a little more.This should have a minimal effect on the final response. You may also wish to turn the woofer's around 90 degrees so the truncated edges are closer to the tweeter and bottom.

Name

Part Type

Value

Secondary Value

Per Speaker

Per Pair

C1,C3

Capacitor

8.2

2

4

C2

Capacitor

18

1

2

C4

Capacitor

1

1

2

L1

Inductor

0.44

18 ga.

1

2

L2

Inductor

1.2

15 ga.

1

2

R1

Resistor

3.9

12 W

1

2

R2

Resistor

1.2

12 W

1

2

R3

Resistor

10

12 W

1

2

S1

Tweeter

Vifa

XT25BG60-04

1

S2

Woofer

Peerless

830991

1

Cabinet

Dayton

TW-0.25 or TWC-0.25

(Comes in pairs)

1

Bass Port

1.5"

4" long

1

2

Banana Terminals

(Comes in pairs)

1

Acousta-Stuff

1 lb.

Sonic Barrier

3/4"

3-layer

1 Sheet

If you decide to make your own cabinet, you MUST maintain the baffle dimensions of 190.5mm x 304.8mm or 7.5" x 12" AND you must ensure the internal volume is at least 0.28 cubic feet. The internal volume can be up to around 0.35 cubic feet, so don't sweat that so much.

Wednesday, May 25, 2016

In the post A Cynical Discussion of Speaker Pricing I make the point that commercial high-end speaker pairs cost from 20 to 30 times the driver cost of a single speaker. This is just a little math to make it easier, but it's equivalent to 10 to 15 times the cost of all the drivers in a pair.

In this post I present a slightly more detailed example of this for those of you who are still incredulous. I'll use the Sony AR1. Sony is by no means unique in this field by the way, it's just that the Sony AR1 lends itself particularly well to public scrutiny.

The Sony SS AR1 MSRP to Driver Cost

Here's a good example, the Sony SS-AR1.

All Scanspeak, off-the-shelf drivers, and they don't even bother to use
the top of the line Beryllium tweeter either. Retail is around $27,000.

By the way, I am a huge Scanspeak fan, I think Sony did well to choose them. That's not the point I want to make here though! The real point is that at Madisound a hobbyist would spend around $2,110 in speaker
drivers per PAIR! So, counting the speaker drivers, the markup from
hobbyist to commercial speaker is around 13x. Add in that Sony probably gets a 35% discount and Dave is then exactly right, it's 20:1.

Of course, this doesn't count labor, lumber or crossover components,
usually the crossover is not shown when they spend as little as possible
on them.

Schematic

At this point in the exploration, I've completed the final draft of
the speaker schematic. Having a little more time and energy I was able
to get better measurements and re-think the crossover entirely. The
final schematic is very simple and uses only nine crossover components.

In the end I used nearly symmetrical second order filters for both sections. The tweeter's -6 dB filter point is right at 2kHz but the woofer is rolled off a little earlier, around 1.5kHz.

Let's take a look at the "transfer function" chart, or as XSim calls it, the "electrical response" chart. This plots the difference between the amplifier and each driver and shows us what our filter choices are doing electrically. Remember that these effects are additive to the drivers, they are never independent of the driver response or impedance.

As you might expect, it's pretty simple. The woofer has no change up until around 700 Hz where the low pass filter begins. The super straight line is partly due to the zobel. Notice however that the tweeter level is significantly lower, about 6 dB below. That's because it's much more efficient, and we needed the R1 to bring it down. The ripple you see in the tweeter slope is due to the tweeter's own impedance interacting with the high pass filter.

Tweeter Level

In the chart below the blue trace represents using the recommended 4.2 Ohm resistor. The red trace shows the effects of using an 0.5 Ohm resistor. The point of showing both of these lines is that you may substitute
any resistor between 0.5 Ohms up to 5 Ohms while still maintaining
excellent phase matching, so make yourself happy!

Zobel Network

C3/C4 and R3 are a Zobel network which of course not only smooths out
the frequency response of the woofer but in this case also allows for
near perfect phase matching with the tweeter. In a pinch, any combined value of C3 and C4 between around 8.4 and 9.4 will work, but 9.2uF is the optimal value.

Acoustic Distance

The acoustic center of the woofer and tweeter are offset by only 1.1".
The small 5 1/4" driver is shallower than the 6 1/2" equivalent plus we
are surface mounting it, pushing the woofer towards the listener. Thanks
to this combination we have really struck crossover gold in terms of
phase matching. A quarter inch the other way and this simple design
would have turned out much more difficult.

Part Selection

For your convenience, a Complete Parts List is in another page, but here we discuss choices for the drivers and crossover components here.

Drivers

As with Kirk's design, I'll be using Vifa tweeters and Peerless woofers:

Vifa XT25BG60-04
1" Dual Ring Radiator Tweeter, $35. Please do not attempt to use the
smaller, and only slightly less expensive Vifa XT25TG30-04, it lacks the
low-end extension. This driver is also sold for more money under the
Scanspeak brand. If you think you spy the
tweeter in some megabucks speakers, you aren't wrong. It's the same
unit, or often the next model down from this one.

The $45 fiberglass driver could be inexactly substituted by the the Peerless 830656 paper cone woofer
which cost around $20 each. In combination with the cheapest possible
crossover capacitors you will get to around a $400 price point. Of
course, the biggest savings is to build the cabinets yourself.

Crossover Components

I present a few different crossover grades below. Regardless of the choice of crossover caps and resistors I always recommend Jantzen air coil as the starting point. L1 should be 18 gauge, L2
should be 15 gauge. Do not use a bigger gauge coil on L2! The
temptation is there, but the DCR is part of the design. If you must "mod" the coils, use a small-guage foil coil for L2 such as the Goertz 16 guage 1.2 mH coil available at Madisound or any other coil with a DCR between 0.330 and 0.4 Ohms.

Cheapest Possible

To stick with an absolute bargain build, use with Bennic caps and Dayton
audio grade non-inductive resistors. 10W is close enough if they don't
have 12W. You could save a few more bucks ($10 total) by using bi-polar electrolytic capacitors in the woofer, but please don't.

Frugal Freddie's Compromises

If you want to spend just a little more, I suggest Mundorf MKP ($8) for the
8.2uF cap in the tweeter section.
Use Mills resistors. Audyn and Jantzen are also highly thought of.

Balanced Betty

Betty buys parts that are matched by the price and quality of the drivers. She would suggest Clarity ESA cap in the tweeter section of if you want to stay with all Mundorfs, the Mundorf EVO Aluminum in Oil. Either should be under $20.

Stick with Mills resistors everywhere, and Mundorf MKP caps in the woofer section.To keep costs down she might choose Axon caps in the woofer though.

To the left you can see my own build. I ended up using Clarity for the tweeter, along with mostly Axon caps in the woofer section. To make a boring story short, I happened to have 7.5uF Axon's lying around, so I got 1.8uF Mundorfs to make up the Zobel. All resistors are Mills.

The savvy builder will note that the coils are aligned in the same Z axis. Not to worry, the boards themselves will be mounted at 90 degree angles! One on the bottom of the speaker and the other on the side.

The "OMG Are you nuts?" Build

Use Jupiter copper film caps for the tweeter. Capacitor cost? About $800 per pair of speakers. Hate your kid? Does he/she have an overripe college fund? Do it! OK, I'm kidding, it's completely out of balance. Save this kind of money for your $300 or more tweeters.

Please feel free to experiment with parts you like, can afford, and have available. These are just my personal recommendations. If you find caps you think work really well leave me a comment.

The Scientist Build

This speaker lends itself very well to learning about the sound of capacitors. The reason is the very high quality tweeter and that it uses a single capacitor. If you like the idea of experimenting with capacitors yourself, I'd suggest you wire the crossover so that the tweeter cap is external. Add a second set of banana jacks on the rear, spaced about 2-3" apart and connect the tweeter capacitor there. Now you have a very convenient experimentation lab which would allow you to swap capacitors or add small bypass caps instantly.

Driver Phase Matching

"Phase matching" refers to how well two drivers play together across the band in which they both contribut. I usually use the -20dB level as my cut-off (more or less). You can see in the chart below that this is about 700 Hz to 3kHz. That's actually a pretty broad range brought about by the low crossover slopes. Still, notice that wihin this range the dotted red and green phase lines are so close together.

We
can also see that the phase alignment where they cross 180 degrees is perfect. The
longer the phase angles match the more the drivers will blend in with
each other and disappear. Here's another view of that effect. Let's
compare the normal response with an inverted driver (either one). In theory this is the absolute worst possible alignment:

Not
only do we have a 20 dB dip at the crossover frequency, but look at how
symmetrical and broad it is. Again, that the inverted driver produces this text-book null indicates the LM-1 have
excellent phase matching before, during and after the crossover region. This will allow the drivers to blend in, minimize lobing and comb-filtering as the listeners location changes.